Thermal radiation sensors are well known in the art. When formed in a semiconductor material such as silicon or germanium such sensors may be provided as mechanical structures, for example as a micro electromechanical (MEMS) arrangement, or electromagnetic (EM) radiation sensors such as infra-red (IR) sensors. By using materials such as semiconductors, it is possible to form sensors in one or more layers of the substrate from etching and other semiconductor processing techniques so as to result in a desired configuration. Due to the delicate nature of the sensors and their sensitivity to the surrounding environment it is known to provide a protective cap over the sensor, the cap serving to isolate the environment of the sensor from the ambient environment where the sensor is operable. Examples of such structures are provided in U.S. patent application Ser. Nos. 11/584,466, 11/584,733, 11/584,725, 11/584,121, and 11/045,910 which are co-assigned to the Assignee of the present application
Thermal radiation sensors of the type incorporating resistors operate by measuring the impedance difference between the illuminated resistors and the shielded resistors to calculate the heating caused by incident infrared radiation. Each of the resistors are provided on a substrate and are characterised by having a predetermined Temperature Coefficient of Resistance (TCR). In operation, some of the resistors are illuminated by infrared radiation while others remain shielded from infrared radiation. The temperature of the target is inferred from the difference in the temperature between the illuminated and shielded resistors. The shielded and un-shielded resistors are spatially separated on the substrate, typically a substrate formed from a semiconductor material, and it will therefore be understood that the output of the sensor is taken from measurements at two different locations. While it is possible to assume that the base substrate temperature is of negligible influence, this assumption does not always hold—especially applications requiring high sensitivity. Thermal radiation sensors known heretofore are extremely sensitive to both temperature gradients over spatial distance in semiconductor substrate and to temperature drift over time. In this context and within the present disclosure temperature gradient effects are related to differences in the measured temperature at two spatially separated locations on a substrate. Temperature drift in contrast and within the context of the present disclosure is intended to relate to the result of thermal impedance between the thermal radiation sensor and the semiconductor substrate on which the sensor is located, the thermal impedance introducing a characteristic time lag into the sensor's response to temperature changes in the substrate. This thermal drift sensitivity has an associated time constant, so computational correction requires a parametric model or a lookup table with linear interpolation between points.
As described in co-assigned U.S. patent application Ser. Nos. 11/584,466, 11/584,733, 11/584,725, 11/584,121, and 11/045,910 the content of which are incorporated by way of reference, thermal radiation sensors may be provided in a Wheatstone bridge configuration. Similar to other Wheatstone bridge arrangements, to function there must be a differential across the bridge. In this context, one side of the Wheatstone bridge is illuminated with infrared (IR) radiation while the other side is shielded from IR. As a result, a heat sensitive resistor on one side of the bridge is illuminated by the incoming radiation and the output of this side of the bridge maybe compared to its shielded pair to create an output voltage which is proportional to the difference in the resistance change between the illuminated and shielded resistors in each of the two branches.
Heat sensitivity resistors used in such arrangements are typically thermistors or bolometers having a resistance dependent on absolute temperature. Die temperature variations produce a very large signal compared to any sensed IR input. The use of reference resistors in a Wheatstone bridge configuration compensates for this effect. The effectiveness of this compensation is limited by the matching of TCR between all resistors in the bridge. However, even the best matching possible still leaves significant sensitivity to temperature variation in the underlying semiconductor substrate. Additionally, the thermal impedance between the thermal radiation sensor and the semiconductor die substrate may introduce a time constant to the sensor's response to die temperature change. In an environment where die temperature is changing this time constant introduces an error which cannot be corrected without analysis of the die temperature differential with respect to time.